1. Field of the Invention
The present invention relates to a walking assist device for assisting a user (person) in walking.
2. Related Background Art
Conventionally, the applicant of the present application proposed this type of walking assist device, for example, in Japanese Patent Application Laid-Open No. 2007-54616 (hereinafter, referred to as Patent Document 1) or Japanese Patent Application Laid-Open No. 2007-330299 (hereinafter, referred to as Patent Document 2).
These Patent Documents 1 and 2 disclose a walking assist device including a seat member on which a user is seated in a straddling manner, a pair of left and right foot attachment portions fitted to the left and right feet of the user, respectively, and a pair of left and right leg links interconnecting the seat member and the left and right foot attachment portions, respectively.
In this walking assist device, each leg link includes a thigh frame extending from the seat member via a first joint (hip joint), a crus frame extending from the foot attachment portion via a second joint (ankle joint), and a third joint (knee joint) that interconnects the thigh frame and the crus frame so that the frames freely bend and stretch between the first joint and the second joint. Moreover, an electric motor for driving the third joint is mounted at an end of the thigh frame on the first joint side of each leg link. Further, in a state where the foot attachment portion is in contact with the ground, the electric motor applies a driving torque to the third joint of the leg link in the stretching direction of the leg link. This causes a lifting force to be applied to the user from the seat member and consequently the walking assist device bears a part of the weight of the user.
In this instance, the walking assist device controls the motion thereof as described below. Specifically, a desired total lifting force as a total supporting force (translational force), which is required to support a part of the weight of the user and the weight of the walking assist device on the floor, is distributed to the leg links at a ratio based on the treading forces of the legs of the user measured from the outputs of treading force measurement force sensors provided on the foot attachment portions. This distribution determines the desired values of the supporting forces applied to the leg links from the floor side (the desired shares of the leg links of the desired total lifting force). In this case, the desired values of the supporting forces of the leg links are determined so that the proportion between the desired values of the supporting forces of the left and right leg links is the same as the proportion between the treading forces of the left and right legs of the user. Moreover, supporting forces actually acting on the leg links from the floor side are measured from the outputs of supporting force measurement force sensors, each of which is interposed between the crus frame and the second joint of the corresponding leg link. Further, an output torque of the electric motor is feedback-controlled for each leg link so that a measured value of the supporting force coincides with the desired value. This allows the output torque of each electric motor to be controlled so that the desired lifting force acts on the user (a translational force supporting a part of the weight of the user) from the seat member.
The above conventional walking assist device, however, controls the electric motor in such a way as to directly achieve matching between the desired value and the value of the supporting force measured from the output of the supporting force measurement force sensor interposed between the crus frame and the second joint of each leg link and therefore has a problem described below.
Specifically, the magnitude of the supporting forces acting or the leg links from the floor side is at the maximum equal to the magnitude of the gravity acting on the combined weight, namely the weight of the entire walking assist device plus a part of the weight of the user. On the other hand, the magnitude of the supporting force is almost “zero” in a state where the leg link is a free leg. This requires a wide range (range width) of values of the supporting force required to be measured from the output of the supporting force measurement force sensor. Therefore, it is difficult to measure the supporting force with high resolution and high accuracy.
Further, the supporting force measurement force sensors are provided at places near the lower ends of the leg links. Moreover, the maximum value of the magnitude of the supporting forces acting on the supporting force measurement force sensors becomes so large as to be equal to the magnitude of the gravity acting on the combined weight, namely the weight of the entire walking assist device plus a part of the weight of the user, as described above. Accordingly, the supporting force measurement force sensors are easily affected by forces in the lateral direction (shear direction) due to deformation of the leg link or the like. Therefore, an interference of the forces in the lateral direction easily decreases the measuring accuracy of the supporting forces required for controlling the lifting force (the supporting forces substantially in the vertical direction). Further, the lower ends of the leg links are widely movable relative to the seat member and therefore often come into contact with other objects. These contacts are also causes of decreasing the measurement accuracy of the supporting forces.
The decrease in the measurement accuracy of the supporting forces could lead to inappropriate driving torques, which are applied to the third joints of the leg links by the electric motors, for applying the desired lifting force to the user from the seat member. In another case, the decrease in the measurement accuracy could lead to a difference between the actual lifting force and the desired lifting force, which gives discomfort to the user.